COD players are obsessed with kills and kill/death ratio. If you play the objective your KDR will (usually) suffer and you won't get as many kills as if you ignored the objective and treated Demo/Sabo/Dom/CTF like a TDM. Sabotage is notorious for this - if you get the bomb planted and blow up the objective in the first minute, everyone will be lucky to have 3 kills, so people simply don't plant the bomb and end up with what is effectively a 20-minute TDM. I even heard some people on my team saying "don't plant the bomb" during a game of Sabotage once...
I personally am unable to
not play the objective - even if I try not to cap flags and plant bombs, I find myself doing it anyway.

If nobody's trying to cap crucial flags in Dom or plant/defuse the bombs in Demolition, I will take it upon myself to do the job. It usually ends up with a noobtube in the face 75% of the time, but at least I'm playing the objective and that's what matters.